Seeing the martian crustal dichotomy

Mike Caplinger, Malin Space Science Systems
February 1994

The simplest way to see the martian crustal dichotomy is to take a
globe of Mars (I'm partial to "Hugg-a-Planet Mars", a stuffed globe of
Mars available in many science stores, museum shops, and from the Planetary
Society) and try to orient it so that you see as few large craters as
possible. On "Hugg-a-Planet Mars," you can orient it such that you
can't see any red (which is the color assigned to the cratered terrain
geologic unit). This happens when you're roughly looking at the crater
Milankovic just east of Arcadia Planitia at about 55 N latitude, 150 W
longitude. You will now be looking at almost none of the heavily-cratered
terrain of the southern highlands, and all of the northern lowlands and
the Tharsis Bulge.

You can also see the dichotomy in the topography alone. I've taken a
topographic map of Mars and digitally colored all the low areas with
blues and all the high areas with yellows and reds, and then projected
the images so that we're looking down at latitude 55 north, longitude
150 west and its antipodes:

Northern lowlands

Southern highlands

The red that you can see in the north is mostly the Tharsis and
Elysium "bulges" where hot material pushing up from the interior of Mars
has raised the elevation of the planet, and led to volcanic eruptions that
have further built up the topography. The blue you can see in the south
are the two impact basins Argyre and Hellas. Otherwise, the north is low and
mostly uncratered and the south is high and largely heavily cratered.

The dichotomy can also be seen easily in MARS by Kieffer et al, from
University of Arizona Press. Color Plate 8 shows the relative ages of
the martian surface based on crater distributions. One can easily see
that the northern lowlands are young, and the southern highlands are
old, but that the boundary is tilted with respect to the equator. We
can also see this on our simplified maps of the
larger craters on Mars.

The source of the dichotomy is not known, although there are several
theories. Some suggest that the lowlands were formed by single or
multiple impacts, while others propose that internal processes like
those that formed the continents on Earth are responsible. We simply
need more data to know for sure.